﻿<p>An <em>IfcShellBasedSurfaceModel</em> represents the shape by a set of open or closed shells. The connected faces within the shell have a dimensionality 2 and are placed in a coordinate space of dimensionality 3.</p>
<blockquote class="extDef">NOTE&nbsp; Definition according to ISO/CD 10303-42:1992<br>
A shell based surface model is described by a set of open or closed shells of dimensionality 2. The shells shall not
intersect except at edges and vertices. In particular, distinct faces may not intersect. A complete face of one shell
may be shared with another shell. Coincident portions of shells shall both reference the same faces, edges and vertices
defining the coincident region. There shall be at least one shell. A shell may exist independently of a surface
model.</blockquote>
<blockquote class="note">NOTE&nbsp; Entity adapted from <strong>shell_based_surface_model</strong> defined in ISO
10303-42.</blockquote>
<blockquote class="history">HISTORY&nbsp; New entity in IFC2x.</blockquote>
<p class="spec-head">Informal Propositions:</p>
<ol>
<li>The dimensionality of the shell based surface model is 2.</li>
<li>The shells shall not overlap or intersect except at common faces, edges or vertices.</li>
</ol>